The primary objective of this series of experiments is to more clearly elucidate the mechanism by which the kidney reabsorbs sodium bicarbonate, and the degree to which this process depends on both the luminal and cytoplasmic forms of carbonic anhydrase. This process will be examined by application of both microelectrode (pH, PCO2) and microcalorimetric techniques in standard free-flow rat micropuncture experiments (cortical and papillary nephron segments), and by in vivo perfusion of superficial proximal and distal nephrons. Three specific experiments will be performed: (1) determination of the magnitude and direction of the directly measured disequilibrium pH by a new technique in superficial proximal and distal tubules and in the papillary collecting duct, (2) evaluation of the contribution of luminal carbonic anhydrase to bicarbonate reabsorption, and (3) evaluation of the determinants of the U-B PCO2 gradient during an alkaline diuresis by determination of the disequilibrium pH, and PCO2 in the papillary collecting duct. These studies should allow differentiation between the two proposed mechanisms of bicarbonate reabsorption: 1) H ion secretion, or 2) primary bicarbonate reabsorption in all the segments of the rat nephron accessible to micropuncture.